1. Field of the Invention
The present invention relates generally to programmable attenuators and amplifiers.
2. Description of the Related Art
Programmable gain devices (e.g., programmable attenuators and amplifiers) find use in numerous systems such as receivers, signal processors and digital-to-analog converters.
An exemplary teaching of programmable attenuators is found in U.S. Pat. No. 5,757,220 (which issued May 26, 1998 to Franklin M. Murden, et al. and was assigned to Analog Devices, Inc., the assignee of the present invention). It describes an attenuator that comprises an R-2R ladder network, a plurality of unity-gain, digitally-switched voltage-to-voltage buffers and a fixed gain stage that sets the attenuator's overall gain/attenuation.
The buffers have digital inputs that respond to binary voltage signals to turn them on and off. In general, only one buffer is turned on at a time so that an attenuated signal at its tap point (on the ladder network) is switched through to the fixed gain stage while the other buffers are held off. In disclosed programmable attenuator embodiments, each buffer has a first stage that remains on with unity gain and a second stage is switched on and off in response to a binary code. The unity gain buffers maintain an approximately constant input impedance whether they are turned on or off and this enhances phase matching over a wide range of signal frequency and power.
Although this attenuator structure is directed to tight phase matching between attenuated signals, it requires numerous complex stages (e.g., a plurality of multi-stage buffers in addition to a fixed gain stage) and fails to take advantage of the benefits of differential signal processing.